we should follow http://www.day.com/specs/jcr/2.0/3_Repository_Model.html#3.6.4.3%20From%20DATE%20To 

there is a method in PHPCR to convert types, and i suggest to just use that

    PropertyType::convertType($date, PropertyType::STRING); 

it will give you `Y-m-d tab H:i:s . fake milliseconds P` (so the diff from greenwhich) which is the string format defined by the standard. the fake milliseconds are due to the fact that java timestamps include those milliseconds which we just don't have in php. the rest of jackalope is using that format and parsing it from the backend so it seems to be fine. do you want to try this?